Delivery of therapeutic agents to a mammalian host can frequently be as important as the activity of the drug in providing effective treatment. For the most part, drugs are delivered orally, frequently initially at a dosage below the therapeutic dosage and by repetitive administration of the drug, the dosage is raised to a therapeutic level or a level exceeding the therapeutic level. In many cases, the fact of having a dosage above therapeutic level provides for adverse effects, since most drugs are not only effective for the intended purpose, but frequently have adverse side effects. Various proposals have been made to avoid these problems, such as slow-release capsules, depots, pumps, and the like. These various approaches have numerous short comings for general applications where one wishes to maintain the presence of a therapeutic agent at a therapeutic dosage for an extended period. Invasive procedures are frequently undesirable, requiring surgery for introduction of the delivery device, followed by subsequent removal. Where the delivery device is placed on the skin, the agent must be capable of transport across the skin at the desired rate. Slow release particles have a limited time span and when introduced into the blood stream will be rapidly phagocytosed.
For those therapeutic agents which must be administered by injection, the need to have repetitive injections is particularly undesirable. The need in many cases for self administration is particularly problematical and in many instances may require trained individuals for the administration. There is, therefore, a serious need for methodologies which would allow for extended administration of therapeutic agents, particularly in the blood stream, which can be easily administered and efficacy maintained for extended periods of time.